Endoprostheses are generally connected to the ends of long bones by anchoring a shank of the prosthesis in the cavity of the long bone. If this is to be done without using cement, then, in order to achieve sufficient anchoring stability, there must be substantial correspondence in shape between the prosthesis shank and the bone cavity intended to receive it. For this purpose, a rasp is used whose configuration corresponds with that of the prosthesis shank. After the epiphysis of the bone has been opened, and, if appropriate, after drilling through the epiphyseal spongy substance into the medullary canal of the diaphysis, the rasp is pushed into the bone to the position which the prosthesis shank is intended subsequently to assume. In doing so, the spongy bone is loosened by the rasp action of the instrument and for the most part removed from the bone.
U.S. Pat. No. 4,671,275 discloses a rasp which has an approximately rectangular cross section and is toothed only at the corners. Between these there are smooth, concave surfaces which do not exert any cutting or tearing action, but merely displace the bone material with which they make contact. This avoids the need to pack bone material around the prosthesis shank. The object of the invention is to make available a prosthesis system which consists of prosthesis and rasp and which permits an especially secure fit of the prosthesis shank in the bone. This is achieved by the features of claim 1 and preferably those of the dependent claims.
The invention is based on the observation that the increased strength of that area of the bone cavity generated by the untoothed profile part of the rasp improves the security of the prosthesis fit all the more the closer this area cooperates with the surface of the prosthesis shank. According to the invention, the shank therefore has at least one longitudinal rib arranged in that part of the prosthesis shank corresponding to the untoothed profile part of the rasp. Although it is advantageous for this rib to extend along the entire length of the shank, this is not absolutely essential, as long as a large part of the shank length is involved, preferably greater than one third, particularly preferably greater than two thirds, of the shank length.
The invention recognizes that a particularly secure prosthesis fit is obtained if the abovementioned longitudinal rib of the shank is absent in the profile of the rasp, at least over much of the length of the latter. In this case, the bone cavity created by the rasp in order to receive the shank has no groove at the location where the rib of the shank is situated. Instead, this groove is created only by the shank rib itself. As the shank is being introduced, it cuts like a wedge into the bone material. In this way, a bed is created in the bone cavity for receiving the prosthesis shank, in which bed the prosthesis shank, by virtue of its cross sectional shape, compresses the spongy substance at least in some areas and achieves optimum bone contact, preferably over greater than 70%, more preferably over greater than 80% of the shank surface.
The effect achieved according to the invention is further increased if the untoothed profile part of the rasp is dimensioned smaller than the corresponding part of the shank, with the result that, after implantation of the prosthesis shank, the surface of the bone cavity surrounds the latter with prestressing.
The invention is of particular importance for anchoring of a hip prosthesis which is to be implanted without cement. If the shank of the hip prosthesis is curved in approximation to Shenton""s line, the untoothed profile part should be arranged on the ventral and/or dorsal aspect of the rasp, preferably on both aspects, while the rasp is toothed at least on the medial aspect. In this way, the surface of the bone cavity can be adapted very precisely to the surface shape of the shank in the medial area. First, this is important because the bone surface is fairly irregular there. Second, a relatively close approximation of the shank surface to the hard cortical bone is desired here. Thus, the remaining spongy layer, where the cortical bone has not been reached directly, is already comparatively compact and gives little play for compaction of the spongy bone substance, so that, in areas where pressure is too great, there is a risk of the cortical bone bursting if one were to rely solely on displacement of the spongy substance. For this reason, the toothing on the medial aspect of the rasp should be provided at least in the proximal area thereof. That means a section of about 4 to 7 cm in length on the medial aspect of the shank, measured from the upper end of the shank. The lateral aspect of the rasp is also expediently toothed, at least in the proximal section thereof.
By contrast, it is expedient if the distal section of the rasp is designed without sharp transverse edges on the medial and lateral aspects. This is because it is intended to fulfil a guide function, upon insertion of the rasp into the bone, by means of sliding along the inner surface of the cortical bone, primarily in the diaphyseal area, the aim of this being to avoid unnecessary damage to said cortical bone.
The untoothed areas on the ventral and/or dorsal aspects of the rasp preferably extend along the entire length of the rasp. Both aspects can be provided with the abovementioned longitudinal rib, which likewise is preferably arranged running along essentially the entire length of the shank. It is particularly advantageous if this rib is formed by a pair of grooves enclosing it on both sides, by which means a large contact surface is created for the intimate contact between the prosthesis shank and the compacted bone material. If this rib is absent in the rasp profile, only the groove cross sections are formed thereon, their bottom surfaces being connected essentially directly to each other. This creates, on the ventral and dorsal aspects of the rasp, extensive surfaces which are expediently spaced apart from each other by a distance which increases in a wedge shape from the direction of the distal end, in order to gradually compact the bone material along which the rasp slides as it is being pushed into the bone.
The concept of the invention can be summarized by stating that a prosthesis system consisting of prosthesis and rasp is made available which, in order to create the cavity for receiving the prosthesis, does not require prior excavation of bone material, but instead leaves the bone material in situ, and with which, by means of marked differences in shape between rasp and prosthesis shank, the spongy bone substance is compressed, upon insertion of the prosthesis shank, in such a way that an exercise-stable and secure fit of the prosthesis is achieved with high vital form closure to the bone.